In most current wireless communication systems, nodes in a network may be configured to operate based on a single transmit and a single receive antenna. However, for many of current wireless systems, the use of multiple transmit and/or receive antennas may result in an improved overall system performance. These multi-antenna configurations, also known as smart antenna techniques, may be utilized to reduce the negative effects that multipath and/or signal interference may have on signal reception. Existing systems and/or systems which are being currently deployed, for example, CDMA-based systems, TDMA-based systems, WLAN systems, and OFDM-based systems such as IEEE 802.11 a/g/n, may benefit from configurations based on multiple transmit and/or receive antennas. It is anticipated that smart antenna techniques may be increasingly utilized both in connection with the deployment of base station infrastructure and mobile subscriber units in cellular systems to address the increasing capacity demands being placed on those systems. These demands arise, in part, from a shift underway from current voice-based services to next-generation wireless multimedia services that provide voice, video, and data communication.
The utilization of multiple transmit and/or receive antennas is designed to introduce a diversity gain and/or an array gain and to suppress interference generated within the signal reception process. Such diversity gains improve system performance by increasing received signal-to-noise ratio, by providing more robustness against signal interference, and/or by permitting greater frequency reuse for higher capacity. In communication systems that incorporate multi-antenna receivers, a set of M receive antennas may be utilized to null the effect of (M−1) interferers, for example. Accordingly, N signals may be simulataneously transmitted in the same bandwidth using N transmit antennas, with the transmitted signal then being separated into N respective signals by way of a set of N antennas deployed at the receiver. Systems that utilize multiple transmit and multiple receive antenna may be referred to as multiple-input multiple-output (MIMO) systems. One attractive aspect of multi-antenna systems, in particular MIMO systems, is the significant increase in system capacity that may be achieved by utilizing these transmission configurations. For a fixed overall transmitted power, the capacity offered by a MIMO configuration may scale with the increased signal-to-noise ratio (SNR).
However, the widespread deployment of multi-antenna systems in wireless communications, particularly in wireless handset devices, has been limited by the increased cost that results from increased size, complexity, and power consumption. Providing a separate RF chain for each transmit and receive antenna is a direct factor that increases the cost of multi-antenna systems. Each RF chain generally comprises a low noise amplifier (LNA), a filter, a downconverter, and an analog-to-digital converter (A/D). In certain existing single-antenna wireless receivers, the single required RF chain may account for over 30% of the receiver's total cost. It is therefore apparent that as the number of transmit and receive antennas increases, the system complexity, power consumption, and overall cost may increase.
In the case of a single RF chain with multiple antennas, there is the need to determine or estimate separate propagation channels. A simple method may comprise switching to a first receive antenna utilizing, for example, an RF switch, and estimate a first propagation channel. After estimating the first propagation channel, another receive antenna may be selected and its corresponding propagation channel may be estimated. In this regard, this process may be repeated until all the channels have been estimated. However, switching between receive antennas may disrupt the receiver's modem and may lower throughput. This approach may require additional hardware and may also result in propagation channel estimates at different time intervals. Any mechanisms that may be utilized to compensate for the presence of multiple time-varying propagation channels may also present added complexity and cost to the design and operation of MIMO systems.
A single weight approach may work best for a single path, that is, for flat fading channels, because a single weight may not combine all paths arriving at different delays optimally. To optimally combine each multipath at receiving antennas may require multiple weights at different delays. For example, the same number of weights as multipaths arriving at different delays may be required, which may be more like a complete channel equalization approach. On the other hand, utilizing a single weight may have an average combining effect on multiple paths, with sub-optimal performance. A single weight may not be selected so that an optimized combination of multiple paths may be achieved at the receiving antennas. For example, for a Rayleigh flat fading channel, a single weight solution may result in about a 6 dB gain, while for the channels with many Rayleigh faded paths the gain may be reduced to about 2 dB.
Moreover, multi-path propagation in band-limited time dispersive channels may cause inter-symbol interference (ISI), which has been recognized as a major obstacle in achieving increased digital transmission rates with the required accuracy. ISI may occur when the transmitted pulses are smeared out so that pulses that correspond to different symbols are not discernable or separable. Meanwhile, data received from a desired user may be disturbed by other transmitters, due to imperfections in the multiple access scheme, giving rise to inter-carrier interference (ICI). For a reliable digital transmission system, it is necessary to reduce the effects of ISI and ICI.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.